1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter, particularly to a semiconductor device, a display device, a light-emitting device, a lighting device, an imaging device, a security device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a light source of an imaging device or a driving method thereof.
2. Description of the Related Art
Electronic devices having a camera function and imaging elements such as image sensors have been developed. In particular, a portable electronic device having a camera function is actively developed. The portable electronic device has a display on its front surface. Users take pictures using an image sensor, looking at the large display on the back surface of the electronic device.
Recently, an electronic device including an image sensor not only on the back surface but also on the front surface of the electronic device has been developed. That is, an image sensor and a display are provided on the same surface, and a user's face looking at the display or the like is photographed with the image sensor on the front of the electronic device (see Patent Document 1). In addition, when the electronic device is used as a video phone, a user's face is photographed with the image sensor and the image can be sent to an intended party while viewing an image of the intended party on the display.
Note that the illuminance of the subject is sometimes low in taking with an image sensor. In this case, the subject is illuminated with a light source such as a flash or a strobe to increase the illuminance of the subject, so that images can be taken well (see Patent Document 2). A portable electronic device often includes a flash for illuminating a subject in addition to an image sensor. Patent Document 1 discloses an electronic device in which a display portion has a display function and a lighting function for a subject and these functions can be switched.